In the electrophotographic process used in document copier machines of the transfer type, a photoconductive material is placed around a rotating drum or arranged as a belt to be driven by a system of rollers. The moving photoconductive material is passed under a charge-generating station to place a relatively uniform electrostatic charge, usually several hundred volts, across the entirety of the photoconductive surface. Next the photoconductor is moved to an imaging station where it receives light rays reflected from the document to be copied. Since white areas of the original document reflect large amounts of light, the photoconductive material will be discharged to relatively low voltage levels in white areas while the dark areas will continue to contain high voltage levels even after exposure. In that manner, the photoconductive material is caused to bear a charge pattern which corresponds to the printing, shading, etc. present on the original document.
After receiving the image, the photoconductor rotates to a developing station where a developing material, called toner, is placed on the image. This material may be in the form of a black powder which carries a triboelectric charge opposite in polarity to the charge pattern on the photoconductor. Because of the attraction of the oppositely-charged toner, it adheres to the surface of the photoconductor in proportions related to the shading of the original. Thus, black printing should receive heavy toner deposits, white background areas should receive none, and gray or otherwise shaded portions of the original should receive intermediate amounts. The developed image is then moved to a transfer station where a copy-receiving material, usually paper, is juxtaposed to the developed image on the photoconductor. A charge is then placed on the backside of the copy paper so that when the paper is stripped from the photoconductor, the toner material is held on the paper and removed from the photoconductor. The remaining process steps call for permanently bonding the toner material to the copy paper and cleaning any residual toner left upon the photoconductive material so that it can be reused for a subsequent copy production.
In the cleaning step, it is customary to pass the photoconductor under a preclean charge-generating station to neutralize the charged areas and pass the photoconductor under an erase lamp to discharge any remaining charge. In that manner, the residual toner is no longer held by electrostatic attraction to the photoconductor surface and thus it can be easily removed at a cleaning station.
In order to avoid overburdening the cleaning station, it is customary to remove all charge present on the photoconductive surface outside of the image area prior to the development step. This is usually done by using an interimage erase lamp to discharge photoconductive material between the trailing edge of one image and the leading edge of the next. Also, edge erase lamps are used to erase charge along the edges of the photoconductor outside of the image area. For example, if the original document is 215.9.times.279.4 mm (8.5.times.11 inches) in size, and if a full-sized reproduction is desired, the dimensions of the image on the photoconductor will also be 215.9.times.279.4 mm (8.5.times.11 inches).
Many copy machines have the capability of copying various size documents and reproducing them to full size. It is not uncommon for machines to be capable of copying 203.2.times.254-mm (8.times.10-inch) originals, 215.9.times.279.4-mm (8.5.times.11-inch) originals, 215.9.times.330.2-mm (8.5.times.13-inch) originals and 215.9.times.355.6-mm (8.5.times.14-inch) originals. Because of the different sized originals the interimage and edge erase mechanisms must be controlled to erase only that part of the photoconductor which is not being used to reproduce an image for a particular size paper.
Conventionally, the interimage erase mechanism has been either an incandescent or fluorescent lamp(s) whose full energization is controlled to erase only the correct area on the photoconductor. Additionally, the lamps are covered by shields which direct the illumination to the photoconductor in order to obtain sharp edge delineation of the erased charge on the photoconductor. For edge erase mechanisms, typically incandescent lamps have been used where one lamp may erase to the 215.9-mm (8.5-inch) size, for example, and a second lamp to the 203.2-mm (8-inch) 8-inch size. For both paper sizes, the lamps will be shielded so that sharp cutoff is obtained.
While there has been some experimentation with the use of light-emitting diodes (LEDs), the prior art approach has been too expensive for use in commercial machines. Light-emitting diodes each produce a relatively small quantity of light as compared to other types of incandescent lamps. Consequently, they must be situated in an environment where high efficiency light transmitting apparatus is used. As a result, LEDs have been used with fiber optics to transmit light to the photoconductor of xerographic machines and because of the cost of fiber optics the system has not been practical. As a consequence, it is an object of this invention to provide innovative light-transmitting pipes for channeling light from an LED to a xerographic surface in an economical but efficient manner such that LEDs may be used with photoconductive surfaces in a document copying machine to perform the interimage and edge erase functions.
When a succession of discrete light sources are used, such as an LED array, it is desirable to spread the light from LED to LED so that there will be no gaps in the erasure of the photoconductor. On the other hand, it is necessary to control the spreading of light in a second dimension so as to obtain sharp edge delineation of the erased charge. As a consequence, it is an object of this invention to provide a light-channeling mechanism which propagates light in one dimension while cutting it off in a second dimension.